1. Field of the Invention
The present invention relates to a semiconductor switching device for controlling voltage to be applied to a load.
2. Background Art
A semiconductor switching device has been used, such as an inverter for controlling a rotating speed of a motor, or a half bridge circuit which is provided for an inverter bridge circuit used as, for example, an AC source. A power element such as a power MOSFET or an IGBT (Insulated Bipolar Gate Transistor) is used as a semiconductor element since the semiconductor element used in the semiconductor switching device needs to handle a large current.
A load is subject to being controlled by the semiconductor switching device and is connected to an AC source via the semiconductor switching device. The switching element, which is provided to the semiconductor switching device, performs a switching operation according to a driving signal produced by a control circuit such that desired control of the load is accomplished. A chopping current carrying device disclosed in the Japanese Laid-Open Patent Publication No. 8-172793 is an example of the semiconductor switching device.
For converting a power between DC and AC by using a general-purpose inverter, which is one type of the semiconductor switching device, it is common to prepare a plurality of units each having two switching elements connected in series with a DC source, and the load is connected to a midpoint between the two switching elements. There may arise a problem wherein the DC source is short circuited by the two switching elements (this short circuit of the DC source is called as an arm short circuit, hereinafter), since the two switching elements are switched into an ON state simultaneously due to such as a malfunction.
The arm short circuit is a serious problem because it damages the switching element. To suppress that negative effect, a protection circuit may be provided so as to shut off the switching elements when detecting that a temperature of the switching elements becomes higher than a predetermined temperature due to the arm short circuit. However, the switching elements should be shut off in a very short period of time, such as less than several micro-seconds, after the arm short circuit occurs so as to prevent the degradation of the switching element. There arises a problem of the difficulty in ensuring the safe shut off of the switching elements since shutting off the switching elements in a very short period of time is difficult. Moreover, detecting the arm short circuit in a short period of time and shutting off the switching elements quickly may be difficult depending on the circuit configuration.
For the period of the off state of the switching elements, a diode is often provided to the semiconductor switching device so as to release energy stored in the load that is accumulated during the operation. Such diode is used to release energy stored in the load. However, when the switching elements switched to on state again, there arises a problem of a switching loss caused by the recovery current that is generated in the diode.
Further, there exists a semiconductor switching device in which a plurality of semiconductor elements is connected in parallel with each other so as to handle a large current. When the semiconductor elements are connected in parallel with each other, there arises a problem of increase of the inductance between the chips. Namely, when a plurality of the switching elements and the diodes are mounted on a substrate for the purpose of handling a large current, inductance of a wiring that connects chips is increased.
Basically, the wiring inductance is proportional to a size of the substrate. Thus, the inductance is increased when many switching elements are connected in parallel so as to handle a large current. Further, as the inductance increased, di/dt is also increased. Accordingly, a back electromotive force voltage, which is the product of the inductance and the di/dt, is increased when a lot of switching elements are connected in parallel, thereby causing a problem of lowering the switching speed. To get around this situation, the switching elements and the diodes may be mounted closely adjacent each other to the substrate so as to decrease the wiring inductance. However, when the switching elements and the diodes are mounted closely adjacent each other, there arises a problem of the obstruction of radiation of heat, for example, from the semiconductor elements.